The present invention relates to an epoxy resin-based composition for semiconductor encapsulation which exhibits a low viscosity at a temperature as relatively low as 80xc2x0 C. or lower and is excellent particularly in dischargeability and coatability as well as storage stability, a semiconductor device prepared by using the epoxy resin-based composition and a process for the production of the semiconductor device.
In order to encapsulate semiconductors at TAP (Tape Automated Bonding) process, COB (Chip On Board) process, etc., a liquid encapsulating medium has heretofore been used. Such a liquid encapsulating medium is used at room temperature (25xc2x0 C.). By means of a dispenser or through printing process, semiconductor elements are encapsulated with such a resinous medium to produce semiconductor devices. As such a liquid encapsulating medium there has been generally known an epoxy resin composition comprising a liquid epoxy resin, an acid anhydride-based liquid curing agent, an ordinary curing accelerator and a silica powder.
The foregoing liquid encapsulating medium comprises an acid anhydride-based curing agent as a curing agent and thus can be easily liquefied and fairly discharged and applied. However, the foregoing liquid encapsulating medium is disadvantageous in that it exhibits a high hydroscopicity under high humidity conditions and thus shows a deteriorated reliability in humidity resistance. Further, the foregoing liquid encapsulating medium normally stays liquid and thus exhibits a poor storage stability. Accordingly, the foregoing liquid encapsulating medium shows a drastic resin in viscosity or liable to sedimentation of silica powder during storage at room temperature and thus needs to be frozen to solid or otherwise stored in a special manner.
It is therefore an object of the present invention to provide a resin composition from semiconductor encapsulation which has a long pot life and is excellent not only in storage stability but in dischargeability and coatability, a semiconductor device encapsulated with the resin composition for semiconductor encapsulation and a process for the production of the semiconductor device.
The foregoing object of the invention will become apparent from the following detailed description and examples.
The foregoing object of the invention is accomplished with the first essence of the invention which lies in a resin composition for semiconductor encapsulation comprising the following components (A) to (D), having a viscosity as measured at 25xc2x0 C. of 700 Paxe2x80xa2s or higher or being solid at 25xc2x0 C. and having a viscosity as measured at 80xc2x0 C. of 500 Paxe2x80xa2s or lower:
(A) an epoxy resin;
(B) an acid anhydride-based curing agent;
(C) a latent curing accelerator; and
(D) an inorganic filler.
The second essence of the invention lies in a semiconductor device comprising a semiconductor element provided on a wiring circuit board with a plurality of connecting electrode portions interposed therebetween, the gap between the wiring circuit board and the semiconductor element being sealed with an encapsulating resin layer, wherein the encapsulating resin layer is formed by the foregoing resin composition for semiconductor encapsulation.
The third essence of the invention lies in a process for the production of a semiconductor device comprising a semiconductor element provided on a wiring circuit board with a plurality of connecting electrode portions interposed therebetween, the gap between the wiring circuit board and the semiconductor element being sealed with an encapsulating resin layer, which comprises filling the gap between the wiring circuit board and the semiconductor element with the foregoing resin composition for semiconductor encapsulation, and then curing the resin composition to form the encapsulating resin layer.
The fourth essence of the invention lies in a semiconductor device comprising a semiconductor element provided on a wiring circuit board, the wiring circuit board and the semiconductor element being electrically connected to each other, the semiconductor element being encapsulated by an encapsulating resin layer in such an arrangement that the semiconductor element is incorporated therein, wherein the encapsulating resin layer is formed by the foregoing resin composition for semiconductor encapsulation.
The fifth essence of the invention lies in a process for the production of a semiconductor device comprising a semiconductor element provided on a wiring circuit board, the wiring circuit board and the semiconductor element being electrically connected to each other, the semiconductor element being encapsulated by an encapsulating resin layer in such an arrangement that the semiconductor element is incorporated therein, which comprises placing a semiconductor element on the wiring circuit board, electrically connecting the wiring circuit board and the semiconductor element to each other, supplying the foregoing resin composition for semiconductor encapsulation onto the wiring circuit board on the semiconductor element side thereof, and then curing the resin composition for semiconductor encapsulation to form the encapsulating resin layer.
The sixth essence of the invention lies in a semiconductor product comprising a semiconductor device having a resin encapsulation layer formed with a plurality of connecting electrode portions interposed therebetween, the semiconductor device being provided on a mounting board with the wiring circuit board of the semiconductor device being opposed to the mounting board and the gap between the mounting board and the semiconductor device being sealed with an encapsulating resin layer, wherein the encapsulating resin layer is formed by the foregoing resin composition for semiconductor encapsulation.
The seventh essence of the invention lies in a process for the production of a semiconductor device, which comprises a step of forming a resin layer made of the foregoing resin composition for semiconductor encapsulation on the protruding electrode portion side of a semiconductor wafer having a plurality of semiconductor elements with protruding electrode portions formed thereon to a predetermined thickness in such an arrangement that at least the forward end of the protruding electrode portions is exposed to the exterior of the resin layer, and a step of cutting the semiconductor wafer having a resin layer formed thereon to individual semiconductor elements.
The eighth essence of the invention lies in a process for the production of a semiconductor device, which comprises a step of supplying the resin composition for semiconductor encapsulation onto the whole of a plurality of semiconductor elements provided on a matrix wiring circuit board having individual circuits formed thereon to form a resin layer with the semiconductor elements being incorporated therein, and a step of cutting the matrix wiring circuit board having a resin layer formed thereon with the semiconductor elements being incorporated therein together with the resin layer to every semiconductor element.
The ninth essence of the invention lies in a process for the production of a semiconductor device, which comprises a step of forming a resin layer made of the foregoing resin composition for semiconductor encapsulation on the protruding electrode portion side of a semiconductor wafer having a plurality of semiconductor elements with protruding electrode portions formed thereon to a predetermined thickness, a step of cutting the semiconductor wafer having a resin layer formed thereon to individual semiconductor elements, and a step of contact-bonding a wiring circuit board and the semiconductor element thus cut under heating with the resin layer side of the semiconductor element and the wiring circuit board being opposed to each other so that the two components are electrically connected to each other and the resin layer is melted and cured to form an encapsulating resin layer between the semiconductor element and the wiring circuit board, thereby encapsulating the semiconductor element.
The tenth essence of the invention lies in a process for the production of a semiconductor device, which comprises a step of applying the foregoing resin composition for semiconductor encapsulation to a matrix wiring circuit board having individual circuits formed thereon to form a resin layer thereon, a step of cutting the wiring circuit board having a resin layer formed thereon to individual wiring circuit boards, and a step of contact-bonding semiconductor elements each having a plurality of connecting electrode portions provided thereon and the wiring circuit boards thus cut with the connecting electrode portion side of the semiconductor elements and the wiring circuit boards being opposed to each other so that the two components are electrically connected to each other and the resin layer is melted and cured to form an encapsulating resin layer between the semiconductor elements and the wiring circuit boards, thereby encapsulating the semiconductor elements.
The inventors made extensive studies of an encapsulating material which has a long pot life and is excellent not only in storage stability but in dischargeability and coatability. As a result, it was found that the desired object of the invention can be accomplished by the use of a resin composition comprising an epoxy resin, an acid anhydride-based curing agent, a latent curing accelerator and an inorganic filler and having a predetermined state and viscosity at each of temperatures of 25xc2x0 C. and 80xc2x0 C. Thus, the present invention has been worked out.
In particular, use of a polyfunctional epoxy resin as the epoxy resin is advantageous in that the composition gives a cured resin having an elevated glass transition temperature (Tg) and enhanced heat resistance.
A resin composition for semiconductor encapsulation comprising as the latent curing accelerator a microcapsule type curing accelerator having a core-shell structure in which a core portion made of the curing accelerator is covered by a specific shell portion is advantageous in that it exhibits an extremely prolonged pot life and thus is particularly excellent in storage stability.
Further, a resin composition for semiconductor encapsulation comprising as the inorganic filler a spherical fused silica powder incorporated therein in a specific proportion is advantageous in that it exhibits an excellent fluidity and thus is particularly excellent in dischargeability and coatability.
Embodiments of implication of the present invention will be described hereinafter in detail.
The resin composition for semiconductor encapsulation of the invention comprises an epoxy resin (component A), an acid anhydride-based curing agent (component B), a latent curing accelerator (component C) and an inorganic filler (component D) and exhibits a specific state and viscosity at each temperature of 25xc2x0 C. and 80xc2x0 C.
The foregoing epoxy resin (component A) is not specifically limited. Various epoxy resins can be used. Examples of the solid epoxy resin employable herein include polyfunctional solid epoxy resin, crystalline epoxy resin, bifunctional solid epoxy resin, triglycidyl isocyanurate, bisphenol F type epoxy resin, and bisphenol A type epoxy resin. These solid epoxy resins may be used singly or in combination of two or more thereof. The term xe2x80x9cpolyfunctional solid epoxy resinxe2x80x9d as used herein is meant to indicate a solid epoxy resin having three or more epoxy groups per molecule. Examples of the polyfunctional solid epoxy resin employable herein include tetrafunctional naphthalene type epoxy resin, triphenylmethane type epoxy resin, dicyclopentadiene type epoxy resin, Techmore VG3101L (produced by MITSUI CHEMICAL CORPORATION), and orthocresol novolak type epoxy resin. The term xe2x80x9ccrystalline epoxy resinxe2x80x9d as used herein is meant to indicate a solid epoxy resin which shows many crystalline peaks when subjected to X-ray diffractometry, physically shows a sharp melting point and exhibits a drastically lowered viscosity during melting because there is little or no intermolecular mutual interaction. Examples of the crystalline epoxy resin employable herein include bisphenol type epoxy resin, biphenyl type epoxy resin, and stilbene type epoxy resin. When those having a melting point of 90xc2x0 C. or higher among these crystalline epoxy resins are used, two or more of these crystalline epoxy resins are preferably used for the reason that the resulting fluidity at a temperature of 80xc2x0 C. or lower is improved.
Among the foregoing epoxy resins (component A), the crystalline epoxy resin may be, e.g., Type GK-4137 epoxy resin (commercially available from NIPPON STEEL CORP.), Type GK-5079 (commercially available from NIPPON STEEL CORP.), and Type YDC-1312 (commercially available from Tohto Kasei Corp.). Type GK-4137 epoxy resin is represented by the following chemical formula (2). Type GK-5079 epoxy resin is represented by the following chemical formula (3). Type YDC-1312 epoxy resin is represented by the following chemical formula (4). 
Among the foregoing crystalline epoxy resins, the biphenyl type epoxy resin is represented by the following general formula (5): 
wherein R3 to R6 may be the same or different and each represent a hydrogen atom or C1-5 alkyl group.
Examples of the C1-5 alkyl group represented by R3 to R6 in the foregoing general formula (5) include straight-chain or branched lower alkyl group such as methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, sec-butyl group and tert-butyl group. R3 to R6 may be the same or different.
As the foregoing biphenyl type epoxy resin there can be used a mixture of a biphenyl type epoxy resin having a structure represented by the following general formula (6) wherein all R3 to R6 are methyl group and a biphenyl type epoxy resin having a structure represented by the following general formula (7) wherein all R3 to R6 are hydrogen atom in almost the same amount: 
The foregoing epoxy resin to be used as component A preferably has an epoxy equivalent of from 140 to 270 g/eq and a softening point of from 50xc2x0 C. to 100xc2x0 C. or a melting point of from 40xc2x0 C. to 150xc2x0 C., particularly an epoxy equivalent of from 150 to 220 g/eq and a softening point of from 60xc2x0 C. to 80xc2x0 C. or a melting point of from 60xc2x0 C. to 130xc2x0 C.
The acid anhydride-based curing agent (component B) to be used in combination with the component A is not specifically limited so far as it acts as a curing agent for the epoxy resin (component A). Various acid anhydrides can be used. An acid anhydride may be used in combination with any of various phenolic resins, amines, phthalic acids, and the like so far as the use of such an optional curing ingredient does not defeat the desired object of the invention. Examples of the acid anhydride-based curing agent (component B) include hexahydrophthalic anhydride, methylhexahydrophthalic anhydride, tetrahydrophthalic anhydride, phthalic anhydride, and trimetallitic anhydride. These acid anhydrides may be used singly or in combination of two or more thereof.
The acid anhydride to be used as the component B preferably has an equivalent of from 100 to 200 , and is more preferably one which has an equivalent of from 150 to 180 and is liquid at ordinary temperature.
The epoxy resin (component A) and the acid anhydride-based curing agent (component B) are mixed together in such a proportion that the amount of the curing agent is preferably from 0.6 to 1.4 equivalents, more preferably from 0.7 to 1.1 equivalent, to the epoxy resin.
A preferred combination of the epoxy resin (component A) and the acid anhydride-based curing agent (component B) comprises, for example, a bisphenol F type epoxy resin [particularly trade name YDF-8170 (produced by Tohto Kasei Co., Ltd.)] and methylhexahydrophthalic anhydride [methylated HHPA (e.g., Rikacid MH-700, produced by Shin Nihon Rika K.K.)] from the standpoints of curability, heat resistance, and flowability.
When incorporated together with the components A and B in the resin composition for semiconductor encapsulation, the foregoing latent curing accelerator (component C) acts to cause the resin composition to show a viscosity increase of 10 times or less after 72 hours of aging in a 50xc2x0 C. atmosphere (viscosity measured at 80xc2x0 C.). An example of the latent curing accelerator is a microcapsule type curing accelerator having a core-shell structure in which a core portion made of one or more selected from various curing accelerators is covered by a shell portion comprising as a main component a polymer having a structural unit represented by the following general formula (1) wherein the reactive amino group present in the shell portion is blocked. The resin composition for semiconductor encapsulation comprising such a microcapsule type curing accelerator incorporated therein exhibits extremely prolonged pot life and thus is excellent particularly in storage stability. Even when the use of an ordinary curing accelerator in a small amount causes the resin composition to show a viscosity increase of 10 times or less, normally 1 to 3 times, the curing accelerator is regarded as a latent curing accelerator. 
wherein R1 and R2, which may be the same or different, each represents a hydrogen atom or a monovalent organic group.
The curing accelerator to be encapsulated as a core portion in the foregoing microcapsule type curing accelerator is not specifically limited so far as it acts to accelerate curing reaction. Known curing accelerators can be used. Those which stay liquid at room temperature are suitable from the standpoint of workability in microencapsulation or the characteristics of the resulting microcapsules. The term xe2x80x9cliquid at room temperaturexe2x80x9d as used herein is meant to indicate that the curing accelerator stays liquid at room temperature (25xc2x0 C.) itself or the curing accelerator stays solid at room temperature but can be liquefied by dissolving or dispersing in an arbitrary organic solvent.
Examples of the curing accelerator to be encapsulated include amine-based curing accelerator, imidazole-based curing accelerator, phosphorus-based curing accelerator, and phosphorus-boron-based curing accelerator. Specific examples of these curing accelerators include imidazole derivatives such as 2-methylimidazole, 2-ethylimidazole, 2-ethyl-4-methylimidazole, 2-isopropylimidazole, 2-dodecylimidazole, 2-undecylimidazole, 2-heptadecylimidazole, 2-phenylimidazole, 1-benzyl-2-methylimidazole, 1-benzyl-2-phenylimidazole, and 1-cyanoethyl-2-methylimidazole, alkyl-substituted guanidines such as ethylguanidine, trimethylguanidine, phenylguanine and diphenylguanidine, 3-substituted phenyl-1,1-dimethylureas such as 3-(3,4-dichlorophenyl)-1,1-dimethylurea, 3-phenyl-1,1-dimethylurea and 3-(4-chlorophenyl)-1,1-dimethylurea, 3-phenyl-1,1-dimethylurea and 3-(4-chlorophenyl)-1,1-dimethylurea, imidazolines such as 2-methylimidazoline, 2-phenylimidazoline, 2-undecylimidazoline and 2-heptadecylimidazoline, monoaminopyridines such as 2-aminopyridine, amineimides such as N,N-dimethyl-N-(2-hydroxy-3-allyloxypropyl)amine-Nxe2x80x2-lactamide, organic phosphorus-based compounds such as ethylphosphine, propylphosphine, butylphosphine, phenylphosphine, trimethylphosphine, triethylphosphine, tributylphosphine, trioctylphosphine, triphenylphosphine, tricyclohexylphosphine, triphenylphosphine/triphenylborane complex and tetraphenylphosphonium tetraphenylborate, diazabicycloalkenes such as 1,8-diazabicyclo[5,4,0]undecene-7, and 1,4-diazabicyclo[2,2,2]octane. These curing accelerators may be used singly or in combination of two or more thereof. Preferred among these curing accelerators are imidazole-based compounds and organic phosphorus-based compounds from the standpoint of ease in production of curing accelerator-containing microcapsules and handling.
The polymer comprising as a main component a polymer having a structural unit represented by the foregoing general formula (1) can be obtained by the polyaddition reaction of polyisocyanates with polyvalent amines or the reaction of polyisocyanates with water.
As the foregoing polyvalent isocyanates there can be used any compounds having two or more isocyanate groups per molecule. Specific examples of these polyvalent isocyanates include diisocyanates such as m-phenylenediisocyanate, p-phenylenediisocyanate, 2,4-tolylenediisocyanate, 2,6-tolylenediisocyanate, naphthalene-1,4-diisocyanate, diphenylmethane-4,4xe2x80x2-diisocyanate, 3,3xe2x80x2-dimethoxy-4,4xe2x80x2-biphenyldiisocyanate, 3,3xe2x80x2-dimethyldiphenylmethne-4,4xe2x80x2-diisocyanate, xylylene-1,4-diisocyanate, 4,4xe2x80x2-diphenylpropanediisocyanate, trimethylenediisocyanate, hexamethylenediisocyanate, propylene-1,2-diisocyanate, butylene-1,2-diisocyanate, cyclohexylene-1,2-diisocyanate and cyclohexylene-1,4-diisocyanate, triisocyanates such as p-phenylenediisothiocyanate, xylylene-1,4-diisothiocyanate and ethylidinediisothiocyanate, tetraisocyanates such as 4,4xe2x80x2-dimethyldiphenylmethane-2,2xe2x80x2,5,5xe2x80x2-tetraisocyanate, adduct of hexamethylenediisocyanate with hexanetriol, adduct of 2,4-tolylenediisocyanate with Brenzcatechol, adduct of tolylenediisocyanate with hexanetriol, adduct of tolyenediisocyanate with trimethylolpropane, adduct of xylylenediisocyanate with trimethylolpropane, adduct of hexamethylenediisocyanate with trimethylolpropane, and isocyanate prepolymers such as trimer of aliphatic polyvalent isocyanate such as triphenyldimethylenetriisocyanate, tetraphenyltrimethylene tetraisocyanate, pentaphenyltetramethylene pentaisocyanate, lysine isocyanate and hexamethylene diisocyanate. These compounds may be used singly or in combination of two or more thereof.
Preferred among the foregoing polyvalent isocyanates from the standpoints of film-forming properties in microencapsulation and mechanical strength is at least one trivalent isocyanate prepolymer selected from (X) an adduct of tolylene diisocyanate with trimethylolpropane and (Y) an adduct of xylylene diisocyanate with trimethylolpropane. Furthermore, triphenyldimethylene triisocyanate also can be used as a preferred polyvalent isocyanate.
On the other hand, as the polyvalent amines to be reacted with the polyvalent isocyanates there can be used any compounds having two or more amino groups per molecule. Specific examples of these compounds employable herein include diethylenetriamine, triethylenetetramine, tetraethylene pentamine, 1,6-hexamethylenediamine, 1,8-octamethylene diamine, 1,12-dodecamethylenediamine, o-phenylenediamine, m-phenylenediamine, p-phenylenediamine, o-xylylenediamine, m-xylylenediamine, p-xylylenediamine, menthanediamine, bis(4-amino-3-methylcyclohexyl)methane, isophoronediamine, 1,3-diaminocyclohexane, and spiroacetal-based diamine. These compounds may be used singly or in combination of two or more thereof.
The reaction of the polyvalent isocyanates with water causes hydrolyzation of the polyvalent isocyanates resulting in the production of an amine that is then reacted with unreacted isocyanate groups (so-called self-polyaddition reaction) is produce a polymer comprising as a main component a polymer having a structural unit represented by the general formula (1).
As the polymer constituting the shell portion (wall membrane) there may be used a polyurethane-polyurea having a urethane bond in common with each other produced from the polyvalent isocyanate and a polyvalent alcohol in combination.
As the foregoing polyvalent alcohol there can be used any of aliphatic alcohol, aromatic alcohol and alicyclic alcohol. Examples of the polyvalent alcohol employable herein include catechol, resorcinol, 1,2-dihydroxy-4-methylbenzene, 1,3-dihdyroxy-5-methylbenzene, 3,4-dihdyroxy-1-methylbenzene, 3,5-dihydroxy-1-methylbenzene, 2,4-dihydroxyethylbenzene, 1,3-naphthalenediol, 1,5-naphthalenediol, 2,7-naphthalenediol, 2,3-naphthalenediol, o,oxe2x80x2-biphenol, p,pxe2x80x2-biphenol, bisphenol A, bis-(2-hydroxyphenyl)methane, xylylenediol, ethylene glycol, 1,3-propylene glycol, 1,4-butylene glycol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, 1,8-octanediol, 1,1,1-trimethylolpropane, hexanetriol, pentaerythritol, glycerin, and sorbitol. These compounds may be used singly or in combination of two or more thereof.
The foregoing microcapsule type curing accelerator can be prepared through, e.g., the following three steps.
[1st step]
The curing accelerator as a core component is dissolved of finely dispersed in a polyvalent isocyanate as a raw material of wall (shell) to form an oil phase. Subsequently, the oil phase thus formed is dispersed in the aqueous medium (aqueous phase) containing a dispersion stabilizer in the form of oil drop to prepared an O/W (oil phase/aqueous phase) emulsion. Subsequently, in the O/W emulsion was dissolved a polyvalent amine to cause interfacial polymerization of the polyvalent amine with the polyvalent isocyanate in the oil phase to initiate polyaddition reaction. Alternatively, the O/W emulsion is heated so that the polyvalent isocyanate in the oil phase reacts with water at the interface with the aqueous phase to produce an amine that subsequently initiate self-polyaddition reaction. Thus, microcapsules comprising as a shell portion (wall membrane) a polyurea-based polymer, preferably a polyurea having a structural unit represented by the general formula (1) are prepared to obtain a microcapsule dispersion.
On the other hand, when a solid curing accelerator is dissolved in an organic solvent to form a core component, an S/O/W (solid phase/oil phase/aqueous phase) emulsion is obtained. In this type of an emulsion, the curing accelerator is lipophilic. A hydrophilic curing accelerator can be difficultly formed into this type of an emulsion. In this case, however, the solubility of the curing accelerator can be properly adjusted to form an O/O (oil phase/oil phase) emulsion or S/O/O (solid phase/oil phase/oil phase) emulsion which then undergoes interfacial polymerization.
The organic solvent for dissolving the curing accelerator therein is not specifically limited so far as it stays liquid at room temperature. In practice, however, one which does not dissolve the shell portion (wall membrane) therein needs to be selected. Specific examples of the organic solvent employable herein include organic solvents such as ethyl acetate, methyl ethyl ketone, acetone, methylene chloride, xylene, toluene and tetrahydrofuran, and oils such as phenylxylylethane and dialkylnaophthalene.
[2nd step]
In the microcapsule dispersion obtained at the 1st step is then dissolved or dispersed a blocking agent. In this procedure, the addition of the blocking agent can be effected after the removal of the dispersion stabilizer or unreacted amine from the aqueous phase by centrifugal separation or the like to advantage.
[3rd step]
The microcapsule dispersion which has been subjected to blocking of amino group with a blocking agent at the 2nd step can be subjected to centrifugal separation, filtration or the like to remove excessive blocking agent, and then dried to prepare a powdered microcapsule type curing accelerator.
Examples of the dispersion stabilizer to be added to the aqueous medium (aqueous phase) at the 1st step include water-soluble high molecular compounds such as polyvinyl alcohol and hydroxymethyl cellulose, and surface active agents such as anionic surface active agent, nonionic surface active agent and cationic surface active agent. Alternatively, hydrophilic inorganic colloidal materials such as colloidal silica and viscous mineral may be used. The amount of such a dispersion stabilizer to be added is preferably predetermined to be from 0.1 to 10% by weight based on the weight of the aqueous phase.
The blocking agent to be used at the 2nd step is not specifically limited so far as it is a compound reactive with amino group. Examples of such a compound employable herein include compounds which react with amino group to form a covalent bond such as epoxy compound, aldehyde compound, acid anhydride, ester compound and isocyanate. Further examples of such a compound include acidic compounds which undergo neutralization reaction with amino group to form a salt such as organic carboxylic acids (e.g., acetic acid, formic acid, lactic acid, oxalic acid and succinic acid, organic sulfonic acids such as p-toluenesulfonic acid, 2-naphthalenesulfonic acid and dodecylbenzenesulfonic acid), phenol compounds, inorganic acids such as boric acid, phosphoric acid, nitric acid, nitrous acid and hydrochloric acid, and solid materials having an acidic surface (e.g., silica, aerogil). Preferred among these compounds are the acidic compounds, which can be used to effectively block the amino group present on and inside the wall membrane. Particularly preferred among these acidic compounds are formic acid and organic sulfonic acids.
The foregoing blocking agent is added in an amount equimolar with the amino group present on and inside the wall membrane. In practical, when as the blocking agent there is used as acidic compound, the acidic material (acidic compound) is added to the dispersion which has just been subjected to microcapsulization (interfacial polymerization) so that the pH value of the dispersion is adjusted from basicity to acidity, preferably a pH of 2 to 5. Thereafter, the dispersion may be subjected to centrifugal separation, filtration or the like to remove excessive acidic compound.
In the process for the production of a microcapsule type curing accelerator comprising the foregoing 1st to 3rd steps, the 2nd step may also involve the passing of the microcapsule dispersion through an acidic cation exchange column by which the unreacted free amine can be removed or the remaining amino group can be neutralized.
The average particle diameter of the microcapsule type curing accelerator thus obtained is not specifically limited. In practice, however, it is preferably predetermined to a range of from 0.05 to 500 xcexcm, more preferably from 0.1 to 30 xcexcm from the standpoint of uniformity in dispersion. The shape of the microcapsule type curing accelerator is preferably sphere but may be epplipsoid. In the case where the microcapsules are not uniform in particle diameter, e.g., where the microcapsules are ellipsoidal or flat rather than spherical, the value obtained by simply averaging the longest diameter and the shortest diameter is defined as average particle diameter.
The amount of the curing accelerator to be encapsulated in the microcapsule type curing accelerator is preferably predetermined to be from 10 to 95% by weight, particularly from 30 to 80% by weight based on the total amount of microcapsules. In other words, when the amount of the curing accelerator to be encapsulated falls below 10% by weight, the curing reaction time is too long to give sufficient reactivity. On the contrary, when the amount of the curing accelerator to be encapsulated exceeds 95% by weight, the thickness of the resulting wall membrane is too small to provide the core portion (curing agent) with sufficient isolatability and mechanical strength.
The ratio of the thickness of the shell portion (wall membrane) to the particle diameter of the microcapsule type curing accelerator is preferably predetermined to a range of from 3 to 25%, particularly from 5 to 25%. In other words, when the foregoing ratio falls below 3%, the resulting microcapsules cannot be provided with a sufficient mechanical strength against the shear developed at the kneading step during the production of epoxy resin composition. On the contrary, when the foregoing ratio exceeds 25%, there is a tendency that the curing accelerator encapsulated is insufficiently released.
The amount of the latent curing accelerator (component C) to be incorporated is preferably from 0.1 to 40 parts by weight (hereinafter abbreviated as xe2x80x9cpartsxe2x80x9d), more preferably from 5 to 20 parts, per 100 parts of the acid anhydride-based curing agent (component B). This is because when the amount of the latent curing accelerator incorporated is smaller than 0.1 part, the composition has too low a curing rate, leading to reduced strength, and because amounts thereof exceeding 40 parts result in too high a curing rate and this may impair flowability.
The amount of the latent curing accelerator (component C) to be incorporated is preferably predetermined to a range of from 0.1 to 40 parts by weight (hereinafter abbreviated as xe2x80x9cpartsxe2x80x9d), particularly from 5 to 20 parts based 100 parts of the above acid anhydride-based curing agent (component B). In other words, when the amount of the latent curing accelerator to be incorporated falls below 0.1 parts, the curing rate is so low that the resulting strength is liable to be lowered. On the contrary, when the amount of the latent curing accelerator to be incorporated exceeds 40 parts, the curing rate is so high that the fluidity is liable to be impaired.
In the invention, as the latent curing accelerator to be used as component C there may be used a commercially available microcapsule type curing accelerator besides the curing accelerator-containing microcapsules so far as the desired object cannot be impaired. Examples of these commercially available microcapsule type curing accelerators include MCE-9957 (comprising as wall membrane methyl methacrylate; produced by NIPPON KAYAKU CO., LTD.), and Novacure produced by Asahi Ciba Co., Ltd. (HX-3748, 3741, 3742, HX-3921HR, HX-3941HP). Even curing accelerators other than microcapsule type curing accelerator may be used such as those having a reduced catalytic activity such as dicyan diamide and Fujicure FXR-1030 and FXE-1000 produced by Fuji Chemical Industry Corp. or if they are ordinary curing accelerators and added in a small amount to give a reduced catalyst activity.
The inorganic filler (component D) to be used as the components A to C is not specifically limited. Various inorganic fillers may be used. Examples of these inorganic fillers include silica, clay, gypsum, calcium carbonate, barium sulfate, alumina, beryllium oxide, silicon carbide, silicon nitride, and aerogil. These compounds may comprise an electrically-conductive particulate material such as nickel, gold, copper, silver, tin, lead and Bi incorporated therein. Preferred among these compounds is spherical silica powder, particularly spherical fused silica powder. The average particle diameter of the inorganic filter is preferably from 0.01 to 60 xcexcm, more preferably from 0.1 to 15 xcexcm. The term xe2x80x9csphericalxe2x80x9d as used herein is meant to indicate that the sphericity is 0.85 or more on the average as measured by a Type FPIA-100 flow type particle image analyzer (produced by SYSMEX INC.).
The content of the inorganic filler (component D) is preferably from 50 to 92% by weight, more preferably from 60 to 88% by weight, based on the whole resin composition for semiconductor encapsulation. This is because when the content of the inorganic filler is lower than 50% by weight, the composition gives a cured resin having an increased coefficient of linear expansion and tending to have a stress, and because when the content thereof exceeds 92% by weight, the composition tends to have reduced flowability and show impaired dischargeability and applicability.
Further, the resin composition for semiconductor encapsulation of the invention may comprise other additives incorporated therein besides the components A to D if necessary.
Examples of the other additives employable herein include fire retardant, wax, alcohols such as ethyl alcohol and methyl alcohol, glycols such as polypropylene glycol, polyethylene glycol, and glycerol, leveling agent, anti-foaming agent, flux, pigment dye, silane coupling agent, and titanate-based coupling agent. Examples of the silane coupling agent employable herein include xcex3-mercaptopropyl trimethoxysilane, xcex3-glycidoxypropyl methyl diethoxysilane, xcex2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane, xcex3-methacryloxypropyl trimethoxysilane, and amino group-containing silane. These silane coupling agents may be used singly or in combination of two or more thereof.
Examples of the first retardant employable herein include novolak type brominated epoxy resin, and brominated bisphenol A type epoxy resin, metallic compounds such as antimony trioxide, antimony pentaoxide, magnesium hydroxide and aluminum hydroxide, and phosphorus-based compounds such as red phosphorus and ester phosphate. These fire retardants may be used singly or in combination of two or more thereof.
Also usable besides those fire retardants is a polyhedral metal hydroxide represented by the following general formula (8). This metal hydroxide has a polyhedral crystal form, and differs from conventional metal hydroxides having a hexagonal platy crystal form or having a thin platy crystal form, like flaky ones. Specifically, the polyhedral crystal form is obtained from a metal hydroxide having a high crystal growth rate not only in the length and width directions but in the thickness direction (c-axis direction), e.g., one having a platy crystal form, by causing the crystals to grow in the thickness direction (c-axis direction) into particulate crystals which are more three-dimensional and nearly spherical. For example, the metal hydroxide has a crystal form which is nearly dodecahedral, nearly octahedral, nearly tetrahedral, etc.
Ml-xQx(OH)2xe2x80x83xe2x80x83(8)
In formula (8), M is at least one metal atom selected from Mg, Ca, Sn, and Ti; Q is at least one metal atom selected from Mn, Fe, Co, Ni, Cu, and Zn; and x is a positive number of from 0.01 to 0.5.
A metal hydroxide having such a polyhedral crystal form can be obtained, for example, by regulating various conditions in a metal hydroxide production process so as to grow crystals not only in the length and width directions but in the thickness direction (c-axis direction). Thus, a metal hydroxide having the desired polyhedral crystal form, e.g., a nearly dodecahedral, nearly octahedral, or nearly tetrahedral form, can be obtained. Usually, the metal hydroxide is a mixture of particles having such crystal forms.
Typical examples of the metal hydroxide having a polyhedral crystal form include Mgl-xNix(OH)2[0.01 less than xc3x97 less than 0.5] and Mgl-xZnx(OH)2[0.01 less than xc3x97 less than 0.5]. Examples of commercial products of these metal hydroxides include Echomag, produced by Tateho Chemical Industries Co., Ltd.
The metal hydroxide having a polyhedral crystal form has an aspect ratio of generally from 1 to 8, preferably from 1 to 7, more preferably from 1 to 4. The term xe2x80x9caspect ratioxe2x80x9d herein means the ratio of the length of the major axis of the metal hydroxide to that of the minor axis thereof. When the aspect ratio thereof exceeds 8, the epoxy resin composition containing this metal hydroxide is reduced in the effect of having a low viscosity upon melting.
Examples of the wax employable herein include a higher fatty acid, a higher fatty acid ester, a higher fatty acid calcium, and an amide-based compound. These waxes may be used singly or in combination of two or more thereof.
The resin composition for semiconductor encapsulation of the invention may comprise a silicone oil, silicone rubber, synthetic rubber or reactive diluent incorporated therein besides the above-described other additives to reduce the resulting stress. The resin composition for semiconductor encapsulation of the invention may comprise an ion trap agent such as hydrotalcite and bismuth hydroxide for the purpose of enhancing the reliability in the humidity resistance reliability test.
The resin composition for semiconductor encapsulation of the invention can be prepared, e.g., in the following manner. In some detail, the foregoing components A to D and optionally other additives are mixed, and then kneaded under heating in a kneader such as universal agitator so that it is melt-mixed. Subsequently, the mixture is cooled to room temperature (approx. 25xc2x0 C.) to produce the desired resin composition for semiconductor encapsulation. In order to adjust the fluidity of the resin composition for semiconductor encapsulation, an organic solvent may be added. Examples of the organic solvent employable herein include toluene, xylene, methyl ethyl ketone (MEK), acetone, and diacetone alcohol. These organic solvents may be used singly or in combination of two or more thereof.
The resin composition for semiconductor encapsulation thus obtained must be solid at 25xc2x0 C. or have a viscosity as measured at 25xc2x0 C. of 700 Paxe2x80xa2s or higher and must have a viscosity as measured at 80xc2x0 C. of 500 Paxe2x80xa2s or lower. Especially preferably, the resin composition is solid at 25xc2x0 C. or has a viscosity as measured at 25xc2x0 C. of 700 Paxe2x80xa2s or higher, and has a viscosity as measured at 80xc2x0 C. of 300 paxe2x80xa2s or lower. In other words, when the viscosity of the resin composition for semiconductor encapsulation falls below 700 Paxe2x80xa2s at 25xc2x0 C., the resulting resin composition exhibits deteriorated storage stability, when the viscosity of the resin composition for semiconductor encapsulation exceed 500 Paxe2x80xa2s at 80xc2x0 C., the resulting resin composition exhibits deteriorated dischargeability and coatability, and thus, in both cases the resulting resin composition cannot satisfy the desired initial requirements.
The viscosity of the resin composition for semiconductor encapsulation at 25xc2x0 C. and 80xc2x0 C. can be measured by an E type viscometer as follows.
[Viscosity at 25xc2x0 C.]
For the measurement of viscosity, a Type RE80U viscometer comprising a cone rotor having a size of 3xc2x0 and R7.7 produced by Toki Sangyo Co., Ltd. was used. The sample was pretreated at a cone rotor rotary speed of 1 rpm for 1 minutes at 25xc2x0 C., and then allowed to stand at 0.1 rpm for 10 minutes at 25xc2x0 C. before measurement.
[Viscosity at 80xc2x0 C.]
For the measurement of viscosity, a Type RE80R viscometer produced by Toki Sangyo Co., Ltd. was used. For the sample having a viscosity of less than 100 Paxe2x80xa2s, a cone rotor having a size of 3xc2x0xc3x97R14 was used. For the sample having a viscosity of 100 Paxe2x80xa2s or more, a cone rotor having a size of 3xc2x0xc3x97R7.7 was used. The sample was pretreated at a cone rotor rotary speed of 1 rpm for 1 minute at 80xc2x0 C., and then allowed to stand at 0.5 rpm for 10 minutes at 80xc2x0 C. before measurement.
The production of a semiconductor device from the resin composition for semiconductor encapsulation of the invention is accomplished by various known methods. In mounting by flip chip, COB, grabtop, cavity fill, etc., the resin composition for semiconductor encapsulation which has been heated (to a temperature of from about 40xc2x0 C. to 90xc2x0 C., preferably from about 60xc2x0 C. to 80xc2x0 C.) is supplied by means of a dispenser so that the semiconductor element is potted. The resin composition is then heated and cured to form an encapsulating resin layer. Thus, a semiconductor device can be produced. Alternatively, a solid or semi-solid resin composition for semiconductor encapsulation may be directly adhered or applied to the semiconductor element, etc., and then heated and cured to form an encapsulating resin layer. Thus, a semiconductor device can be produced. The mounting may be effected in vacuo.
Among the foregoing processes for the production of a semiconductor device, flip chip mounting will be further described hereinafter with reference to side fill encapsulation process, press bump encapsulation process and printing encapsulation process.
[Side fill encapsulation process]
A wiring circuit board having semiconductor elements provided thereon with a plurality of connecting electrode portions interposed therebetween is prepared. The gap between the wiring circuit board and semiconductor elements which have been previously heated (to a temperature of from about 40xc2x0 C. to 130xc2x0 C., preferably from 60xc2x0 C. to 100xc2x0 C.) is then filled with the resin composition for semiconductor encapsulation which has been heated (to a temperature of from about 40xc2x0 C. to 90xc2x0 C., preferably from about 60xc2x0 C. to 80xc2x0 C.) and injected by means of a dispenser. The resin composition thus injected is then heated and cured to form an encapsulating resin layer. In this manner, a semiconductor device can be produced by flip chip mounting.
Alternatively, a solid or semi-solid resin composition for semiconductor encapsulation which has not previously been heated may be directly applied onto the semiconductor elements or to the area in the vicinity thereof, and then heated and cured to form an encapsulating resin layer in the gap between the semiconductor elements and the wiring circuit board.
The production of a semiconductor by side fill encapsulation process may be effected in vacuo. As the apparatus for the production of such a semiconductor device in vacuo there may be used a Type MBC-V Series produced by Musashi Engineering Co., Ltd. The foregoing production of a semiconductor device in vacuo may be accomplished by differential pressure filling method which comprises filling of the gap between the wiring circuit board and the semiconductor elements in vacuo with the resin composition for semiconductor encapsulation which has been injected by means of a dispenser and subsequent filling the gap with the resin composition at atmospheric pressure.
[Press bump encapsulation process]
The resin composition for semiconductor encapsulation which has been heated (to a temperature of from about 40xc2x0 C. to 90xc2x0 C., preferably from 60xc2x0 C. to 80xc2x0 C.) is applied onto the wiring circuit board by means of a dispenser so that the semiconductor elements are potted. Thereafter, press bump connection process using a flip chip bonder or the like is effected to carry out the electrical connection between the semiconductor elements and the wiring circuit board and the formation of an encapsulating resin layer at the same time. In this manner, a semiconductor device can be produced by flip chip mounting.
Alternatively, a solid or semi-solid resin composition for semiconductor encapsulation which has not previously been heated may be directly applied to the semiconductor elements or wiring circuit board, followed by press bump connection process. In this manner, the connection between the semiconductor elements and the wiring circuit board and the formation of an encapsulating resin layer can be effected at the same time.
The production of a semiconductor device by press bump encapsulation process may be effected in vacuo.
Instead of potting by means of a dispenser, printing may be effected to apply the resin composition if possible, followed by press bump connection process by flip chip bonder or the like which effects the electrical connection between the semiconductor elements and the wiring circuit board and the formation of an encapsulating resin layer at the same time. In the application by printing, the printing atmosphere may be entirely heated or the mask or squeegee may be partly heated (to a temperature of from about 40xc2x0 C. to 100xc2x0 C.).
[Printing encapsulation process]
A wiring circuit board having semiconductor elements provided thereon with a plurality of connecting electrode portions interposed therebetween is prepared. The resin composition for semiconductor encapsulation which has been heated (to a temperature of from about 40xc2x0 C. to 90xc2x0 C., preferably from about 60xc2x0 C. to 80xc2x0 C.) is then dropped into the gap between the wiring circuit board and semiconductor elements which has previously been heated (to a temperature of from about 40xc2x0 C. to 130xc2x0 C., preferably from about 60xc2x0 C. to 100xc2x0 C.) by means of a dispenser so that printing encapsulation is effected to form an encapsulating resin layer. In this manner, a semiconductor device can be produced by flip chip mounting.
The printing encapsulation process is preferably accomplished by the use of a Type VPE-100 Series vacuum printing encapsulation apparatus using vacuum differential pressure (produced by Toray Engineering Co., Ltd.) because air bubbles can hardly enter into the encapsulating resin layer.
Alternatively, a solid or semi-solid resin composition for semiconductor encapsulation which has not previously been heated may be directly applied to a stage or squeegee so that printing encapsulation can be effected.
On the other hand, among the foregoing processes for the production of a semiconductor device, the cavity fill process for the production of a semiconductor device will be further described hereinafter.
A wiring circuit board having semiconductor elements provided thereon in such an arrangement that the semiconductor elements are electrically connected thereto through a bonding wire or the like is prepared. The resin composition for semiconductor encapsulation which has been heated (to a temperature of from about 40xc2x0 C. to 90xc2x0 C., preferably from about 60xc2x0 C. to 80xc2x0 C.) is then applied to the wiring circuit board and semiconductor elements which has previously been heated (to a temperature of from about 40xc2x0 C. to 130xc2x0 C., preferably from about 60xc2x0 C. to 100xc2x0 C.) by means of a dispenser so that the semiconductor elements are potted. The resin composition is then heated and cured so that an encapsulating resin layer is formed to encapsulate the semiconductor elements. In this manner, a cavity fill type of semiconductor device can be produced.
Alternatively, a solid or semi-solid resin composition for semiconductor encapsulation which has not previously been heated may be directly adhered or applied to the semiconductor elements, and then heated and cured to form an encapsulating resin layer in such an arrangement that the semiconductor elements are encapsulated.
The production of a semiconductor device by the foregoing encapsulation method may be effected in vacuo. As the apparatus for the production of such a semiconductor device in vacuo there may be used a Type MBC-V Series produced by Musashi Engineering Co., Ltd.
Other processes for the production of a semiconductor device will be further described hereinafter. A wiring circuit board having semiconductor elements provided thereon in such an arrangement that the semiconductor elements are electrically connected thereto through a bonding wire or the like is prepared. The resin composition for semiconductor encapsulation which has been heated (to a temperature of from about 40xc2x0 C. to 90xc2x0 C., preferably from about 60xc2x0 C. to 80xc2x0 C.) is then supplied on the wiring circuit board and semiconductor elements which have previously been heated (to a temperature of from about 40xc2x0 C. to 130xc2x0 C., preferably from 60xc2x0 C. to 100xc2x0 C.) by printing or the like, and then heated and cured to form an encapsulating resin layer in such an arrangement that the semiconductor elements are encapsulated. In this manner, a cavity fill type of semiconductor device can be produced.
The production of a semiconductor device by printing encapsulation process may be effected in vacuo. In the procedure of producing a semiconductor device in vacuo, printing encapsulation in vacuo may be followed by the rise in the atmospheric pressure that causes the removal of voids from the resin composition for semiconductor encapsulation. Finish printing may be then effected under these conditions.
The semiconductor device is then placed on a mounting board (mother board) to produce a semiconductor product. In some detail, the semiconductor device is mounted on the mounting board (mother board) with a plurality of connecting electrode portions being provided interposed therebetween. At the same time, the gap between the mounting board and the semiconductor device is filled with the resin composition for semiconductor encapsulation of the invention which is then heated and cured to form an encapsulating resin layer. In this manner, a semiconductor product is produced.
The method for heating and curing the resin composition for semiconductor encapsulation is not specifically limited. In practice, however, a heating method using a convection dryer, IR reflow furnace, hot plate or the like can be used.
Examples of the method for filling the gap between the mounting board and the semiconductor device with the resin composition for semiconductor encapsulation of the invention include the same methods as described with flip chip mounting among the processes for the production of a semiconductor device mentioned above, i.e., side fill encapsulation process, press bump encapsulation process, printing encapsulation process, etc. The resin composition for semiconductor encapsulation may comprise an electrically=conductive particulate material such as nickel, gold, silver, copper, tin, lead and Bismuth dispersed therein to form ACT (Anisotropic Conductive Film) or ACP (Anisotropic Conductive Paste) which is then used for flip chip mounting. In other embodiments, the resin composition for semiconductor encapsulation may be applied to the wiring circuit board as a damming material or may be used as an adhesive for bonding the wiring circuit board to the heat radiator or a die-bonding agent.
The production of a semiconductor device comprising the resin composition for semiconductor encapsulation of the invention which has been provided on a semiconductor wafer or matrix wiring circuit board can be accomplished by various known methods.
The production of a semiconductor device will be described with reference to the case where the resin composition for semiconductor encapsulation of the invention is applied to a semiconductor wafer having a plurality of semiconductor elements with protruding electrode portions formed thereon. In some detail, the resin composition for semiconductor encapsulation which has been heated (to a temperature of from about 40xc2x0 C. to 90xc2x0 C., preferably from about 60xc2x0 C. to 80xc2x0 C.) is applied to the protruding electrode portion side of the semiconductor wafer by means of a dispenser to form a resin layer made of the resin composition for semiconductor encapsulation to a predetermined thickness. The formation of a resin layer made of the resin composition for semiconductor encapsulation to a predetermined thickness is effected in such a manner that at least the forward end of the protruding electrode portion is exposed to the exterior of the resin layer. Subsequently, the semiconductor wafer having a resin layer formed thereon is cut to produce a semiconductor device.
The formation of a resin layer made of the resin composition for semiconductor encapsulation can be carried out by printing through an opening in the mask.
The resin layer thus formed may be in any state so far as it is eventually thermoset. The heating and curing of the resin layer may be effected before or after cutting of the semiconductor wafer.
On the other hand, the resin composition for semiconductor encapsulation is supplied onto the whole of a plurality of semiconductor elements mounted on a matrix wiring circuit board having individual wiring circuits formed thereon to form a resin layer in such an arrangement that the semiconductor elements are encapsulated. Subsequently, the resin layer is thermoset to encapsulate the plurality of semiconductor elements with the resin composition. The plurality of semiconductor element to produce semiconductor devices.
The resin layer thus formed may be in any state so far as it is eventually thermoset. The heating and curing of the resin layer may be effected before or after cutting of the semiconductor elements.
As the method for forming the resin layer made of the resin composition for semiconductor encapsulation there may be used a method using a dispenser, a method involving printing through an opening in the mask or the like similarly to the method previously mentioned.
Alternatively, the resin composition for semiconductor encapsulation is supplied onto the protruding electrode portion side of a semiconductor wafer having a plurality of semiconductor elements with protruding electrode portions formed thereon to form a resin layer to a predetermined thickness. The semiconductor wafer having a resin layer formed thereon is then cut to individual semiconductor elements. Subsequently, the wiring circuit board and the semiconductor elements are contact-bonded to each other under heating with the resin layer side of the semiconductor elements thus separated and the plurality of wiring circuit boards being opposed to each other so that they are electrically connected to each other. At the same time, the resin layer is melted and cured to form an encapsulated resin layer between the semiconductor elements and the wiring circuit boards so that the semiconductor elements are encapsulated. In this manner, a semiconductor device is produced.
Alternatively, the resin composition for semiconductor encapsulation is supplied onto a matrix wiring circuit board having individual wiring circuits formed thereon to form a resin layer. The wiring circuit board having a resin layer formed thereon is then cut to individual wiring circuit boards. Subsequently, semiconductor elements each provided with a plurality of connecting electrode portions and the wiring circuit boards thus separated are contact-bonded to each other under heating with the connecting electrode portion side of the semiconductor elements and the wiring circuit boards being opposed to each other so that they are electrically connected to each other. At the same time, the resin layer is melted and cured to form an encapsulating resin layer between the semiconductor elements and the wiring circuit boards so that the semiconductor elements are encapsulated. In this manner, a semiconductor device is produced.
As the method for forming the resin layer made of the resin composition for semiconductor encapsulation there may be used a method using a dispenser, a method involving printing through an opening in the mask or the like similarly to the method previously mentioned.
The present invention will be further described in the following examples and comparative examples, but the present invention should not be construed as being limited thereto.
The following ingredients were prepared prior to these Examples.
[Epoxy resin a1]
Crystalline epoxy resin represented by the following chemical formula (2) (GK-4137, produced by Nippon Steel Chemical Co., Ltd.; epoxy equivalent, 174 g/eq; melting point, 70xc2x0 C.). 
[Epoxy resin a2]
Bisphenol F type epoxy resin (Epo Tohto YDF-8170, produced by Tohto Kasei Co., Ltd.,; liquid at 25xc2x0 C.; epoxy equivalent, 158 g/eq).
[Epoxy resin a3]
Bisphenol A type epoxy resin (Epikote 1001W, produced by Yuka Shell Co., Ltd.; solid at 25xc2x0 C.; epoxy equivalent, 405 g/eq).
[Epoxy resin a4]
Crystalline epoxy resin represented by the following chemical formula (3) (GK-5079, produced by Nippon Steel Chemical Co., Ltd.; epoxy equivalent, 190 g/eq; melting point, 78xc2x0 C.). 
[Acid anhydride]
Methylhexahydrophthalic anhydride (methylated HHPA, Rikacid MH-700, produced by New Japan Chemical Co., Ltd.)
[Curing accelerator c1]
A microcapsule type curing accelerator was prepared in accordance with the method previously mentioned. In some detail, 11 parts of an adduct of 3 mols of xylylene diisocyanate and 1 mol of trimethylolpropane and 4.6 parts of an adduct of 3 mols of tolylene diisocyanate and 1 mol of trimethylolpropane were uniformly dissolved in a mixture of 7 parts of triphenyl phosphine and 3.9 parts of ethyl acetate as curing accelerators to prepare an oil phase. Separately, an aqueous phase comprising 100 parts of distilled water and 5 parts of a polyvinyl alcohol was prepared. To the aqueous phase thus prepared was then added the oil phase prepared as above. The mixture was then subjected to emulsification by means of a homomixer to form an emulsion. The emulsion thus formed was then charged into a polymerization reactor equipped with a reflux tube, an agitator and a dropping funnel.
On the other hand, 10 parts of an aqueous solution containing 3 parts of triethylenetetramine were prepared. The aqueous solution thus prepared was then put into the dropping funnel provided in the foregoing polymerization reactor. The aqueous solution was dropped into the emulsion in the reactor where interfacial polymerization took place at a temperature of 70xc2x0 C. for 3 hours to obtain an aqueous suspension of microcapsule type curing accelerator. Subsequently, the aqueous suspension was subjected to centrifugal separation to remove the polyvinyl alcohol, etc. from the aqueous phase. To the residue were then added 100 parts of distilled water. The mixture was then subjected to dispersion to obtain a suspension.
To the suspension was then added formic acid to adjust the pH value of the system to 3. In this manner, a microcapsule type curing accelerator in which the amino group present on and inside the wall membrane had been blocked with formic acid was prepared. The microcapsule type curing accelerator thus obtained was repeatedly subjected to centrifugal separation and rinsing, and then dried to isolate the microcapsule type curing accelerator as a free-flowing powder. The average particle diameter of the microcapsule type curing accelerator was 2 xcexcm. The ratio of the shell thickness to the particle diameter of microcapsules was 15%. The encapsulated amount of triphenylphosphine was 30% by weight based on the total amount of the microcapsule type curing accelerator.
[Curing accelerator c2]
MCE-9957 produced by NIPPON KAYAKU CO., LTD.
[Curing accelerator c3]
2-Ethyl-4-methylimidazole (Curezol 2E4MZ, produced by Shikoku Chemicals Corp.)
[Inorganic filler d1]
Spherical fused silica powder (SE-2100, produced by Admatechs Co., Ltd.; average particle diameter: 0.56xcexcm)
[Inorganic filler d2]
Spherical fused silica powder (FB-48X, produced by DENKI KAGAKU KOGYO K.K.; average particle diameter: 15 xcexcm)